Industrial robots are usually constructed with an architecture that is heavy and rigid. The consequences of a collision between the robot and its environment may therefore be disastrous, even at low speed: firstly there is the risk of damaging the environment, which may include fragile articles and human beings, and secondly the robot itself may be damaged in a collision with an element of the environment stronger than itself, for example a concrete wall. The problem therefore arises of limiting the consequences of collisions that may occur if a robot of high inertia moves quickly in an environment that is not static.
One solution that has been proposed is monitoring the working space of the robot by means of a multitude of fixed video cameras. Those video cameras combine data concerning the environment of the robot and data concerning the movement of the robot in that environment. The data are then processed by a computer connected to the video cameras in order to determine and predict the imminence of a collision with the environment. That solution has several drawbacks. Firstly, image processing is time-consuming, and that limits the speed at which the robot may move. Secondly, the algorithms are complex and therefore they are of only moderate reliability. Thirdly, those algorithms require considerable calculation resources. Moreover, the field of view of a video camera may be accidentally obstructed, for example if the lighting is turned down or off completely, or else in the opposite situation of excessive lighting. Video cameras are also complex pieces of equipment with a non-negligible risk of failure.
Another solution that has been proposed consists in using various devices that are capable of mechanically detecting contact between the robot and the environment. In one variant, use is made of a six-axis force sensor disposed at the base of the robot. The difficulty of such a solution is to distinguish low environment/robot contact forces that are precursors of a collision from high forces that result from the inherent dynamics and mass of the robot, not to mention the inherent noise of the sensor. In order to be able to withstand the weight of the robot, the sensor must be over-specified in comparison with the contact forces. Consequently, to obtain sufficient sensitivity it is necessary to filter the signals coming from the sensor, which is time-consuming and greatly increases the time needed to detect a collision. The same applies to systems for detecting forces on the body of the robot by measuring the motor currents, as the effect of friction in the robot must then also be taken into account.
Another solution consists in equipping the robot with bumpers associated with pressure sensors. However, when the robot is in motion, the inertia of a bumper and the stiffness of the pressure sensors make fine measurements uncertain, which may limit the sensitivity of the system when it is most needed at very beginning of the collision.
Another variant uses sensors responsive to contact forces distributed over the whole of the body of the robot. That kind of solution has the drawback that, when the robot is in motion, contact between the robot and the environment is detected too late to be able to prevent a collision.
A further solution distributes proximity sensors over the body of the robot. The distribution of the proximity sensors over the body of the robot represents a compromise between the detection distance and the number of sensors. A very short detection distance requires a large number of detectors, which is difficult to implement. In contrast, if the number of sensors is low, the detection distance is relatively large, and so the sensors may accidentally detect parts of the robot other than that to which they are fixed.
Another solution, known from Document WO 2008/066575, consists in covering a portion of the holding member of a robot with a touch-sensitive sensor, the touch-sensitive sensor including electrodes and a deformable layer of ionic liquid or gel associated with the electrodes so that a current flowing between two electrodes is proportional to the thickness of the deformable gel or liquid layer at the location of said electrodes. However, such a sensor proves complex to install, in particular when covering a large area of the robot, notably because of the need to insert each electrode through the wall of the robot, the need to connect each electrode to the input of the multiplexer placed inside the robot by means of a wire, and the need to seal the deformable layer. Covering a large area cannot be envisaged because of the difficulty of obtaining a deformable layer that is uniform over the whole of the sensor. Furthermore, on surfaces that are not plane or that are moving, gravity deforms the liquid layer of the sensor and acts in different manners thereon, causing variations of resistance between the electrodes in the absence of contact.